The present invention relates to low voltage, high continuous dc current vacuum switches, which are typically used as electrical shunt elements for electrolytic cell chemical processing systems. In such devices a very high continuous dc current at low voltage is passed through the chemical cell to produce the desired chemical components, such as chlorine, sodium hydroxide, or even a refined metal such as copper or aluminum. Such cells are typically used electrically in series, and it is desirable and necessary to be able to isolate or shunt a single cell from the bank of cells for maintenance and/or chemical recharging. A low voltage electrolytic cell shunting switch is seen in U.S. Pat. No. 4,088,859. When such a low voltage vacuum switch is closed, with the contacts in abutting relationship within the vacuum chamber, the current which would otherwise pass through the electrolytic cell is diverted through the vacuum switch which is typically rated at about 6,000 amperes, at up to about 10 volts dc.
The vacuum switch must be effective to interrupt the high amperage current arc which strikes between the contacts as they are opened to divert the current back through the electrolytic cell when it is to be put back into operation. The low voltage dc switch is effective to interrupt this high current arc because a given arc voltage is required to sustain an arc in vacuum for such dc applications. This arc voltage is typically about 20 volts dc and is largely a function of the contact materials, but does not significantly vary for materials such as copper, copperbismuth, or tungsten contacts. The low voltage dc switch with a single arc path is incapable of interrupting operation at dc potentials which exceed the arc voltage. This has limited reliable application of the switch with adequate overvoltage margin to those systems that operate at relatively low dc voltages, typically at about 10 volts or less. There is a class of electrolytic cell that operates at between 20 and 50 volts dc, which is above the arc voltage generally required to sustain a dc arc in vacuum. It has been possible to interrupt such a higher voltage circuit by using several individual low voltage vacuum switches in series. This requires multiple external connections of the switches which can be relatively expensive because of the high continuous dc current carrying capability which the bus connectors must be capable of sustaining. Numerous connections can give rise to high contact resistance which should be avoided.
In ac vacuum interrupters it has been known to employ a multiple-break vacuum-type circuit interrupter as seen in U.S. Pat. No. 3,405,245. An electrically floating center contact was shown in conjunction with a single, or with two movable end contacts to form a multi-break interrupter for the purpose of boosting the withstand voltage capability of the device. An ac vacuum interrupter is effective to interrupt the vacuum arc between the contacts, because the arc current is constantly oscillating and passes through multiple zero current cycles as the contacts are moved apart. At some distance of contact separation, the dielectric strength of the vacuum is sufficient to extinguish the arc, and so long as the withstand voltage for restriking an arc is below an acceptable value, the arc will remain extinguished. This is a different interruption phenomenon than the use of the arc voltage drop in a low voltage dc vacuum switch, since in such switches there is no current oscillation through a current zero.